As a conventional optical recording medium, for example, there is an optical disk, such as a CD-ROM and a DVD-ROM. In such an optical disk, an uneven row of pits is formed on a transparent substrate which is made of polycarbonate or the like. On the substrate, a metal reflection film is formed which is made of Al or the like. From a side of a surface opposite to a surface on which this metal reflection film is formed, a beam of light is applied to the metal reflection film which is an information recording surface. Thereby, information is reproduced.
Such an optical recording medium has been widely used in which information is recorded and reproduced by applying a beam of light. Thus, expectations have become greater of heightening its recording density from now on. In recent years, a variety of optical disks has been developed which can reproduce large-capacity audio-visual data or digital data. For example, research and development for a high-density ROM optical disk is now going on, in which a density of an optical disk which has a diameter of 12 centimeters is expected to become higher to a storage capacity of 23.3 to 30 gigabits.
On the other hand, a DVD ROM recording medium is provided with a security technique, specifically, a technique of preventing someone from illicitly using and copying recorded information or from doing such an act. As that security technique, a BCA (or burst cutting area) area is provided where medium identification information, which is used to identify each recording medium individually, is overwritten in a bar-code pattern. In this BCA area, when an optical recording medium is manufactured, medium identification information which differs for each optical recording medium is recorded, and if necessary, a key of cryptograph or a key of decoding is recorded.
For example, Japanese Patent Laid-Open No. 10-233019 specification discloses that a metal reflection film of an optical disk on which a row of pits is formed as main data is partially removed by laser trimming, and modulated data is recorded individually. Thus, medium identification information is recorded which is used to protect against illicitly using and copying, or such an act.
However, in order to heighten the above described density, a pitch between tracks has to be narrowed, or a shortest pit of a row of pits needs to be shortened. Besides, with respect to a high-density optical disk, at least 23.3 GB data is recorded on a 12 cm-diameter optical disk. Therefore, it has been determined that if on a substrate used for such an optical disk, a metal reflection film is formed which is made of an Al alloy material having a film thickness of 50 to 70 nm so that it can be used in a DVD ROM optical disk, that deteriorates quality of a reproduced signal.
This is because a metal reflection film seems to be difficult to form at a bottom of a minute pit about 0.2 μm long. Thus, the shorter a pit becomes, the deeper and the smaller it tends to be. Accordingly, as a metal reflection film for the above described high-density ROM optical disk, a metal reflection film which is used in a DVD ROM optical disk could not be used as it is.
In addition, when a DVD ROM optical disk is manufactured, medium identification information is recorded, using a medium-identification-information recording apparatus which is provided with a YAG (yttrium aluminum garnet) laser. However, even if the medium identification information is recorded in a bar-code pattern using this medium-identification-information recording apparatus, on an area where pits are not formed in a high-density ROM optical disk or on a row of pits which is recorded at a track pitch of 0.74 μm, which is the same as in the DVD ROM optical disk, then a pattern could not be formed. Or, reproduction noise of the medium identification information became louder, and thereby, an adequate defocus margin could not be secured.
This is because in a high-density ROM optical disk, a metal reflection film is thinner than that of a DVD ROM optical disk. Or, material of a metal reflection film in use is different, and thus, heat capacity necessary until the metal reflection film reaches its melting point is largely different. Accordingly, a conventional medium-identification-information recording apparatus provided with a YAG could not be used as it is when a high-density ROM optical disk is manufactured.